The present invention relates to sheets that can be used as displays that selectively transmit or reflect light.
Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information, however magnetically written data is not visible.
A structure is disclosed in PCT/WO 97/04398, entitled xe2x80x9cElectronic Book With Multiple Display Pagesxe2x80x9d which is a thorough recitation of the art of thin, electronically written display technologies. Disclosed is the assembling of multiple display sheets that are bound into a xe2x80x9cbookxe2x80x9d, each sheet provided with means to individually address each page. The patent recites prior art in forming thin, electronically written pages, including flexible sheets, image modulating material formed from a bi-stable liquid crystal system, and thin metallic conductor lines on each page.
Fabrication of flexible, electronically written display sheets are disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent indium-tin-oxide (ITO) conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential applied to opposing conductive areas operate on the liquid crystal material to expose display areas. The display uses nematic liquid crystal materials which ceases to present an image when de-energized.
U.S. Pat. No. 5,223,959 discloses a plurality of polymer dispersed liquid crystal material, each having a different dye material of red, green or blue dye material. Differing electrical signals to common electrodes operate on each of the materials to control the state of each type of dyed liquid crystal material. The patent requires the use of conventional nematic liquid crystals with a dye to absorb light. The droplets are chemically treated to be stable in either a clear or a light absorbing state. The sheet requires materials having different response times to electrical signals. The device must be continually driven so that the human eye perceives complementary colors. This arrangement has the disadvantage of requiring continuous, high speed electrical drive because the materials do not maintain their state. The material must be driven to achieve a neutral color density.
U.S. Pat. No. 5,437,811 discloses a light modulating cell having a polymerically dispersed chiral nematic liquid crystal. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal-conic state. The structure has the capacity of maintaining one of the given states in the absence of an electric field.
U.S. Pat. No. 5,847,798 discloses a continuous tone, neutral density display that can be driven between a reflective and light absorbing state. The display uses a single cholesteric material to provide a constant reflectance across the visible light spectrum to provide a neutral color. The material is reflective in the non-visible spectrum and appears clear in the visible spectrum. The display can also be driven in to a stable light scattering state that is uniformly scattering in the visible spectrum. A neutral xe2x80x9cwhitexe2x80x9d state occurs, but the provides a very low reflective intensity.
U.S Pat. No. 4,576,891 discloses encapsulated dye-forming capsules that generate color images in response to pressure. Separate capsules responsive to red, green and blue wavelengths of light and forming colors responsive to those respective wavelengths are selectively hardened and form color images when crushed. Capsules are formed separately and mixed to create a single pressure sensitive layer. The sheets are light written and the dye forming process is irreversible.
Currently, privacy windows are created using the scattering properties of conventional nematic liquid crystals. Such materials require continuous electrical drive to remain transparent.
Accordingly, it is an object of the present invention to provide a neutral density display apparatus permitting continuous tone, neutral density images that are stable in a zero-field condition.
A further object of the invention to provide a neutral density privacy window that is stable in either a tight blocking or light transmitting field in a zero state condition.
Another object is to provide a sheet that can be re-written using electronic means.
Yet another object is to improve optical properties of a sheet by using a single field to operate on a plurality of liquid crystal formulations.
These objects are achieved in a light modulating, electrically responsive sheet comprising:
(a) a transparent substrate;
(b) a transparent, electrically conductive layer formed over the transparent substrate; and
(c) a light modulating layer including a plurality of electrically responsive light modulating materials dispersed in the light modulating layer and having a plurality of optical states.
An advantage of the present invention is the use of reflective characteristic of multiple cholesteric materials to provide light reflection across the visible spectrum.
Sheets made in accordance with the present invention can be used to provide a re-writable image sheet. The present invention uses a plurality of cholesteric liquid crystal materials in a layer which are effective in at least two states, a reflective state and a transmissive state. This invention permits the use of light modulating, electrically responsive sheets with improved reflective efficiency. The sheet can be formed using inexpensive, efficient photographic layer methods. A single large volume of sheet material can be coated and formed into various types of sheets and cards. Displays in the form of sheets in accordance with the present invention are inexpensive, simple and fabricated using low-cost processes.